Many of the critical steps in the fabrication of advanced semiconductor integrated circuits involve processing in a plasma reactor. These steps include etching, chemical vapor deposition (CVD), and physical vapor deposition (PVD). In all these processes, a processing gas flows into the processing chamber, and an electric field excites the gas into a plasma. Particularly in etching and to a lesser extent in CVD, the excited processing gas is very reactive (which is why it is excited into a plasma) and reacts not only with the wafer but also with chamber parts that are exposed to the plasma. As a result, many of the parts in a plasma reactor facing the plasma have presented materials problems. If the plasma significantly reacts with the chamber part, a number of problems may occur. The processing chemistry involving the wafer may be disturbed by the side reactions with the material of the chamber. Some ceramics, such as quartz, are preferentially etched along grain boundaries, and the preferential etching of the inter-granular portions liberates grains from the ceramics. As a result, extraneous particles are formed which settle on the wafer and may drastically reduce the yield of operable chips. Long-term exposure of the chamber part to the plasma may erode the part to such an extent that it fails in its mechanical or electrical function.
A newer generation of plasma reactors is being developed and commercialized which can be characterized as high-density plasma (HDP) reactors. By various means, the ion density of the plasma is increased to levels significantly above prior generations of commercial plasma reactors. The higher density of plasma not only accelerates the processing, but in several applications is required for effectively processing the increasingly smaller features of semiconductor integrated circuits. However, the high-density plasmas have increased the severity of the problems associated with the chamber parts, and prior materials solutions have been shown to be insufficient.
Furthermore, the mechanisms for creating the high-density plasma often require a complexity of chamber design not previously required in commercial semiconductor fabrication equipment.
An example of an advanced plasma etching reactor, particularly useful for oxide etching, is illustrated in the cross-sectional view of FIG. 1. This reactor is simplified from one disclosed by Collins et al. in U.S. Pat. application, Ser. No. 08/648,254, filed May 13, 1996. A wafer 10 is supported on a pedestal 12 facing a processing space 14 within a vacuum chamber. An annular pumping channel 16 formed in a chamber base 18 is pumped by an unillustrated vacuum system. An insulating ring 20 electrically isolates the pedestal 12 from the chamber base 18. A slit valve opening 21 in the chamber base 18 and an associated but unillustrated slit valve allow the wafer 10 to be transferred into and out of the vacuum chamber. The top of the vacuum chamber is formed by a dome 22 composed of silicon, and a silicon ring 24 surrounds the wafer 10. One or more unillustrated gas ports supply, for oxide etching, a fluorine-based etching gas.
The etching gas is excited into a high-density plasma primarily by RF power inductively coupled into the chamber through two concentric helical coils 26, 28 extending above a flat roof 29 of the dome 22. An RF power splitter 30 splits RF power from a source RF power supply 32, for example operating at 2 MHz, between the two coils 26, 28 so as to tailor the RF magnetic field induced within the plasma inside the chamber. A bias RF power supply 34 connected to the pedestal 12 provides a controllable DC self-bias in the plasma sheath adjacent to the wafer 10 for controlling the etching kinetics. The silicon dome 22 is electrically grounded to provide a grounding plane for the chamber.
A number, four in the illustration, of rings 36, 38, 40, 42 composed of a ceramic that is preferably thermally conductive but electrically highly resistive are located on top of the dome roof 29. The two coils 26, 28 are wrapped around two rings 42, 36. A number of radiant lamps 44 are placed in remaining annular channels formed between the rings 36, 38, 40, 42 and control the temperature of the silicon dome 22. Unillustrated radiant heating means also control the temperature of the silicon ring 24 around the wafer 10.
When the chamber is being used to etch a layer of silicon dioxide in the wafer 10 and the etching process must be highly selective against etching underlying silicon or polysilicon, the preferred etching gas is a fluorocarbon, such as CF.sub.4, and the silicon dome 22 and ring 24 are used to scavenge fluorine from the plasma of the etching gas, that is, to remove fluorine, leaving a carbon-rich plasma. As a result, a polymer having a low fluorine content forms on non-oxygen parts of the wafer 10, specifically silicon or silicon nitride parts once the silicon dioxide covering them has been etched away. The low-fluorine polymer protects the non-oxygen parts from etching, thus producing a high etch selectivity. The radiant lamps 44 and other temperature control elements control the temperature of the scavenging dome 22 and ring 24 since the scavenging process is sensitive to temperature.
One of the difficulties with the silicon dome 22 of FIG. 1 is the requirement that it both act as a grounding plane and also pass an RF magnetic field from the coils 26, 28 into the chamber. A good grounding plane requires a high electrical conductivity while an RF wall should have a low electrical conductivity. These conflicting requirements are addressed by striking a balance between the bulk electrical conductivity and the RF skin depth relative to the thickness of the window. Typical resistivities for bulk pieces providing sufficient mechanical strength for a vacuum chamber are in the range of 30 to 200.OMEGA.-cm. Although acceptable results have been achieved, it is desired to better address both requirements. Other difficulties with the silicon dome 22 are that large pieces of silicon of high electrical resistivity are expensive and that it is difficult to control the reproducibility of the ingots from which the pieces are formed. Furthermore, large silicon pieces are prone to chipping and cracking.
Lu et al. in U.S. Pat. application, Ser. No. 08/687,740, filed Jul. 26, 1996, incorporated herein by reference, disclose a dome of a robust, cost-effective material which independently addresses the requirements of the grounding plane and of the RF window. As illustrated for a crown dome 50 in the cross-sectional view of FIG. 2, the dome 50 includes a base part 52 of sintered, preferably hot pressed, silicon carbide and a thin film 54 of silicon carbide extending on the side of the base part 52 exposed to the plasma within the chamber. As Lu et al. explain, the silicon carbide is preferably stoichiometric or nearly so, but its composition may extend through the range of 40 to 60 atomic % of both silicon and carbon. The SiC thin film 54 is preferably formed by chemical vapor deposition (CVD) although other deposition and formation techniques are possible, as is explained by Lu et al. Hirai et al. review CVD formation of silicon carbide in "Silicon carbide prepared by chemical vapor deposition," Silicon Carbide Ceramics - 1: Fundamental and Solid Reaction, eds. Somiya et al., (Elsevier, 1991), pp. 77-98. However, the SiC film may be formed by other well known processes. Silicon carbide is also known to operate as a fluorine scavenger. The base body part 52 includes a number of circular rings 60, 62, 64, 66 providing annular channels 68 as well as one central hole 70 for the two coils 26, 28 and the radiant heaters 44.
A principal advantage of the composite structure is that the bulk, SiC base part 52 can be made fairly resistive so as to not ground out the RF electromagnetic field which the inductive coils couple from one side of the roof to the other. At the same time, the SiC thin film 54 can be made of low-resistivity material that allows effective low-frequency biasing of the thin film 54 by an unillustrated electrical connection and hence of the entire roof. Although such a low resistivity in a bulk part would ground out the electromagnetic field, the thin film 54 may be made thin enough that its thickness is comparable to or less than the RF skin depth, thus not grounding out the electromagnetic field. Examples of thicknesses and resistivities of the roof of the sintered base 52 and of the film 54 are tabulated in TABLE
TABLE 1 ______________________________________ RESISTIVITY RF COUPLING PART (.OMEGA.-cm) EFFICIENCY ______________________________________ Base &gt;10.sup.5 Low RF Loss (18-23 mm) CVD 20-40 Better than Si (4 mm) ______________________________________
The highly resistive base produces negligible RF loss while the moderately conductive but thin CVD film provides 20 to 25% higher RF coupling efficiency than a comparable silicon roof Further advantages of the composite structure are that silicon carbide is a reasonably good thermal conductor and that the two SiC elements 52, 54 have virtually the same coefficients of thermal expansion, as tabulated in TABLE 2 for a number of temperatures
TABLE 2 ______________________________________ Coefficient of Thermal Temperature Expansion (10.sup.-6 /.degree. C.) (.degree. C.) Base CVD Film ______________________________________ 100 3.5 3.7 200 4.0 4.0 300 4.2 4.3 ______________________________________
The closeness of thermal expansion between the sintered base and the CVD film reduces thermal shock. Also, a silicon carbide CVD film 54, which is the only silicon carbide exposed to the processing space, is a very clean material, producing few particles, even in the corrosive fluorine plasma environment of oxide etching. On the other hand, bulk sintered silicon carbide is relatively inexpensive and can be made in even larger sizes for future 300 mm-wafer applications. Finally, sintered silicon carbide exhibits good mechanical characteristics.
Nonetheless, the composite crown dome 50 of FIG. 2 has some disadvantages. The crown dome 50 is relatively large, about 40 cm in diameter and 12 cm in height, and is complexly shaped. Such shapes of silicon carbide can be preformed into near net shape (NNS) by hot pressing, a form of sintering. An example of a NNS base 80 is illustrated in cross section in FIG. 3. In this case, the NNS base 80 is integrally formed with a base plate 82, a base ring 84, and an undefined base roof 86 to be formed later into the rings for the inductive coils and radiant lamps. After the NNS base 80 is formed by sintering, it is machined into final shape, and thereafter the CVD film 54 is deposited to form the crown dome 50 of FIG. 2. The NNS part may assume a more or less complex shape depending on the tradeoff between the amount of machining and the length of the sintering time and cool down required for complex sintered parts. Sintering is a high-temperature and/or high-pressure process, and the size and shape of the crown dome requires a long sintering process including cool down. Furthermore, a single defect at any point in the sintering can ruin an entire dome, thus presenting a large financial risk.
Another problem arises from the fact that the film thickness, of the order of a few millimeters, still must balance grounding resistance and RF skin depth. As a result, the grounding resistance is designed to be only barely adequate. However, during long term processing, the CVD thin film 54 is partially eroded, thus increasing the grounding resistance. Any such variation in grounding resistance introduces a process variation that is best avoided in a reactor intended for continuous production.